1. Field of the Invention
This invention relates to a printed circuit board and a structural arrangement of an electric circuit assembly which has circuit forming elements such as resistors, capacitors, transistors, IC's, etc. disposed on the printed circuit board and which is interconnected by means of conductors, such as a copper foil pattern, to form an electic circuit.
2. Description of the Prior Art
There have been proposed numerous printed circuit boards of the type having conductors on a substrate with land parts provided in necessary parts of the conductors for soldering the circuit elements thereto. The prior art relative to these printed circuit boards includes a method for preventing a conductor on a printed circuit board from short-circuiting, as disclosed in U.S. Pat. Nos. 4,088,824 and 3,433,888, and a method in which conductors are disposed both on the front and reverse surfaces of a printed ciruit board and which have electrical continuity between them as disclosed in U.S. Pat. No. 3,610,811. Meanwhile, in designing a printed circuit board, it is important to appropriately dispose circuit connection wires which three-dimensionally intercross on the electric circuit.
Circuit wires are never allowed to three-dimensionally intercross on a single pattern surface of a printed circuit. While each pattern is thus arranged to have no three-dimensional intercrossing of circuit connection wires, it is inevitable that some wires three-dimensionally intercross. This will be understood, for example, from a printed circuit board 1 shown in FIG. 1 of the accompanying drawings. The circuit board 1 comprises an IC (integrated circuit); land parts A1-A2, B1-B2 C1-C2, . . . which are provided for soldering thereto the lead wires of electric circuit elements; and a conductor pattern 4 which interconnects the land parts or connects with circuit parts which are not shown. In the circuit of FIG. 1, some of the land parts A1 and A2; B1 and B2; C1 and C2; . . . indicated by the same alphabetical reference symbols, sometimes must be interconnected with each other. In one of the methods for interconnection between the land parts, through holes A1, B1, C1, . . . arranged as shown in FIG. 2, a conductor is provided on the reverse side of a double-side printed circuit board. There are also formed the same lands A1-A2, B1-B2, C1-C2, . . . on the reverse side to that which is formed with a conductor, as shown in FIG. 1. On the reverse side, the lands to be interconnected are also connected by the conductor. The front and reverse sides are connected to each other by holes which are provided in the middle of these lands. The printed circuit board must be arranged on both sides using the through holes. However, the circuit wires to be intercrossed do not exist all over the reverse side. Thus, there inevitably is a considerably large wasteful portion on the reverse side. The double-side printed circuit board and the through holes themselves also cause an increase in cost. In many cases, the printed circuit boards used for office equipment and precision apparatus are flexible printed circuit boards (hereinafter called FPC). Compared with a single-side FPC, the cost of a double-side FPC is about twice as high as the former. Furthermore, since the FPC is used in a bent state, it is not readily bendable when conductors are applied to both the front and reverse sides thereof. Another method for connecting conductors is shown in FIG. 3. Referring to FIG. 3, there are wires 6.sub.1 -6.sub.6 which are called jumper wires. The jumper wires 6.sub.1, 6.sub.3, 6.sub.4 and 6.sub.6 are open wires while the jumper wires 6.sub.2 and 6.sub.5 are covered wires. These wires are designed for interconnecting two points. Therefore, where three points are to be interconnected, as shown by reference symbols B and C, points B' and C' must be added as shown in FIG. 3. The covered wires 6.sub.2 and 6.sub.5 are used to avoid a short-circuit which would occur between uncovered wires if they are connected. It is now possible to automatically solder elements and open wires by such soldering means as the reflow soldering method using chip type elements. However, the automatic soldering process is not applicable to these covered wires. Furthermore, the reliability of soldering degrades if the length of these jumper wires is either longer or shorter than a specified length. If the open wires have some metal members located closely, a short-circuit tends to occur. Meanwhile, the covered wires which are soft and have no fixed positions tend to hinder a moving member, if such is located nearby. A short jumper wire may be replaced with a device called a chip jumper which is indicated by a reference numeral 7 in FIG. 4. The chip jumper gives greater soldering reliability than a short jumper wire. However, the use of chip jumpers necessitates an increase in the size of lands. Besides, the number of circuit wires that can be jumped by the chip jumper is limited. In view of these shortcomings, the present invention is directed to providing a method in which use of jumper wires is replaced by the use of an FPC which gives high soldering reliability and ensures safe connection of patterns with reduced probability of a short-circuit or impeded contact with other members.
It is therefore an object of the invention to provide a printed circuit board which permits interconnection between land parts or conductors without intercrossing connection wires.
It is another object of the invention to provide an electric circuit assembly wherein two printed circuit boards are laminated into a package with land parts or conductors interconnected by a novel method which differs from the prior art connecting method.
It is another object of the invention to provide a printed circuit board or an electric circuit assembly which can be manufactured at a low cost and which has a reliable, simple structural arrangement.
It is still another object of the invention to provide an electric circuit assembly which consists of two laminated printed circuit boards and which fcilitates carrying out a test or check-up operation by connecting the circuit assembly to another circuit via a connector after completion of the assembly work thereon.
These and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.